Hydrogen attack is a damage mechanism occurring in steels exposed to high pressure hydrogen at elevated temperatures. Under such conditions, hydrogen atoms diffuse into steels and react with carbides. The reaction leads to formation of methane and, subsequently, intergranular fissuring and losses of material strength and toughness.
Conventional ultrasonic techniques for finding hydrogen attack are insufficient to identify hydrogen damage against factors such as abnormal grain size, inclusions, rough surfaces, curved internal surface, cladding, disbonds, laminar cracks, and variations of the transducer coupling conditions. Neither can they determine the distance of the damage progression.
Previously, inspection for hydrogen attack relied on five techniques:
1. Echo attenuation/Echo Spectrum
This technique measures the loss of backwall echo amplitudes as an indication of hydrogen damage. It has no ability to discriminate hydrogen attack from abnormal grain size, inclusions, laminar cracks, rough surfaces, non-parallel surfaces, internal surface geometry, cladding, and disbonds between cladding and base metal. PA1 This technique measures the amplitude of backscattering signals and uses high backscattering amplitude as the indication of hydrogen damage. It cannot differentiate hydrogen attack from internal flaws such as laminar cracks and inclusions. The validity of the technique also depends on the surface conditions of the calibration material and the material under examination as well as on the pressure applied on the ultrasonic transducer. PA1 This technique measures the shear-to-longitudinal velocity ratio of the entire wall thickness (i.e., including base metal and cladding material) to assess the extent of hydrogen damage. The result is influenced by cladding materials. Also, it cannot identify hydrogen damage less than 15% of the wall thickness. PA1 This technique measures the reduction of creeping wave velocity as the indication of hydrogen damage. It is applicable only to partially damaged steel and only to thin-walled vessels. PA1 The relative change in shear wave velocity is measured and correlated to the extent of hydrogen damage. The technique cannot differentiate hydrogen attack from change of material thickness. Its sensitivity to hydrogen damage is low, similar to that of the velocity ratio technique. PA1 i) differentiate hydrogen attack from large grains, inclusions, laminar cracking, cladding, disbonds, rough surfaces, non-parallel surfaces, surface curvature, transducer couplant, and coupling pressure, and PA1 ii) determine the distance of hydrogen attack progression.
2. Amplitude-based backscatter
3. Velocity ratio
4. Creeping Waves/Time-of-Flight Measurement
5. Pitch-catch mode shear wave velocity